1. Field of Invention
This invention relates generally to fluid servo systems, and more particularly to a servo-controlled fuel injection system for an internal combustion engine to produce optimum fuel-air ratio throughout a broad operating range.
2. Status of Prior Art
The behavior of an internal combustion engine in terms of operating efficiency, fuel economy and emission of pollutants is directly affected both by the fuel-air ratio of the combustible charge and the degree to which the fuel is vaporized and dispersed in air. Under ideal circumstances, the engine should at all times burn 14.7 parts of air to one part of fuel within close limits, this being the stoichiometric ratio. In the actual operation of a conventional system, a ratio richer than stoichiometric is required at idle and slow speeds for dependable operation, whereas leaner than stoichiometric is desirable at higher speeds for reasons of economy. The employment of Lambda oxygen exhaust sensors and feedback controls to maintain the stoichiometric ratio for catalytic control of emissions is at the expense of performance and economy.
Maximum fuel economy and minimum emission of pollutants have heretofore been considered to be mutually exclusive due to the practical limitations of presently available systems. These limitations stem from the inability to "gasify" liquid fuel in air from idle to full speed and power before ignition in the engine. By the term "gasify" is meant fuel that has been dispersed, vaporized and homogenized to a gaslike quality. At or about the stoichiometric ratio of such gasified air-fuel mixtures, the most complete combustion with minimum emissions will result.
Fuel preparation systems for spark-ignition internal combustion engines fall into two general classes: carburetors and fuel injection. These will now be separately considered.
The function of a carburetor is to produce the fuel-air mixture needed for the operation of an internal combustion engine. In the carburetor, fuel is introduced in the form of tiny droplets in a stream of air, the droplets being vaporized as a result of heat absorption in a reduced pressure zone on the way to the combustion chamber whereby the mixture is rendered inflammable. In a conventional carburetor, air flows into the carburetor through a Venturi tube and a fuel nozzle within a booster Venturi concentric with the main Venturi tube. The reduction in pressure at the Venturi throat causes fuel to flow from a float chamber in which the fuel is stored through a fuel jet into the air stream. The fuel is atomized because of the difference between air and fuel velocities.
Although most carburetors today use double and triple Venturis to multiply suction forces, the fixed sizes of these Venturis, usually determined by the mid-range capacity of the engine, gives rise to fuel induction throughout approximately one-half the automotive operating range. The lack of Venturi-carburetion action at idle and slow speeds makes it necessary to introduce fuel downstream of the Venturi by means of the high vacuum developed by partially-open throttle plates. At higher speeds and power, air bleeds are needed to moderate excessive enrichment by the higher Venturi velocities. And under maximum power when the Venturi vacuum is moderate, additional fuel is supplied by means of power jet, stepped needle valves or auxiliary barrels.
Thus existing techniques for regulating the fuel-to-air ratio throughout the existing range from idle to full power represent, at best, a compromise dictated by the above-noted limitations, fuel efficiency being poor at idle, low speeds and high power. Moreover, to overcome acceleration "flat spots" encountered during transitions in driving modes, throttle-actuated fuel pumps are employed to spray additional fuel into the air stream, thereby rendering the system even less efficient.
Modern systems of fuel injection for internal combustion engines produce air-fuel mixtures by means of pressurized fuel nozzles for timed or continuous spray into the air stream. Fuel injection systems are now widely used, for they make possible precise metering and control of the air-fuel ratio over the entire engine operating range, thereby promoting fuel efficiency. Moreover, fuel injection lends itself to the application of after-burning exhaust equipment to reduce the emission of noxious pollutants. Most fuel injection systems in current use are electronically controlled, though mechanical injection systems are also found in some engines.
An electronic fuel injection system includes an electrically-driven fuel pump which supplies and develops the fuel pressure necessary for the system. The fuel is injected by solenoid-operated fuel injection valves into the cylinder intake ports, characterizing such systems as "ported" Electronic Fuel Injection (EFI). The injection valves are controlled by an Electronic Control Unit (ECU) which governs the amount of fuel injected by the length of time they stay open from a constant pressure source. The ECU is provided with data regarding operating conditions and ambient conditions by means of sensors.
Mechanical fuel injection systems are of two general types; those requiring a drive from the engine and those that do not. The engine driven systems comprise a fuel injection pump with an integral governor, the same as that for Diesel engines. The prevailing mechanical injection system needs no direct drive and injects continuously from a constant pressure electric pump fuel supply with regulators that control the amount of fuel injected by varying the fuel pressure to injectors.
The yardstick for determining the quantity of fuel required for all fuel injection systems is the quantity of air drawn in by the engine. Hence an air-flow meter is the important component for controlling the quantity of injected fuel. The function of air-flow sensing and measuring is carried out by various forms of air-flow meters. Presently such meters are either mechanical in the form of a plate movable by air flow in a Venturi-like engine air intake casing, or mechanical-electrical in the form of a "vane" movable in the engine air intake whose movement is transduced to an electric signal by mechanical switching. Also in use are electronic meters such as "hot-wire" and "sonic" sensing for operation in conjunction with microprocessors.
In the Bosch K Jetronic system, a "movable-plate" sensor directly positions a plunger valve in a barrel containing metering slits whereby the plunger opens or closes slits for mor or less fuel flow to individual cylinders from a primary (constant) pressure source. Variation of fuel-to-air ratio is achieved by a variable "control" pressure that biases the plunger movement against air-flow movement--a continuous multi-port injection system.
In the Bosch L Jetronic system, a "vane"-to-electric air-flow meter and various engine and ambient electronic sensors input to an electronic microprocessor (ECU) to control the output of electric-solenoid injector-valves in an electronic multi-port injection system. The other Electronic Fuel Injection (EFI) systems are basically the same, using other forms of electronic air-flow meters.
A mechanical fuel injection system included in an Indianapolis 500 winner in 1970 and still popular with such vehicles makes use of a mechanical air flow meter consisting of a Venturi in the turbo-charged engine intake. Its differential-air pressure (Venturi-vacuum) is applied to a diaphragm whose movement is opposed by differential-fuel pressure of an orifice type jet on an opposing diaphragm, the resultant movement of a valve controlling fuel pressure to individual ported injectors continuously. The non-linear flow characteristics of this system results in poor control at the low end and results in undue enrichment at the high end, thereby rendering this system unusable for passenger and commercial vehicles.
The Abbey series of U.S. Pat. Nos. 4,118,444; 4,187,805; 4,250,856; 4,308,835 and 4,387,685 discloses a unique "floating" Venturi structure that when positioned in the air-intake to the engine produces a pneumatic pressure-differential whose magnitude is linearly proportional to the mass-volume of air flowing therethrough and is effective throughout the entire operating range of the modern high-speed engine. This constitutes a mechanical air-flow meter that provides a pneumatic signal that is applicable to mechanical fuel regulation for fuel-to-air proportioning control.
The above-identified patents, whose disclosures are incorporated herein by reference, also disclose continuous injection systems that utilize the "floating" Venturi meter controlling fuel injection by means of a fuel pressure regulator. They also disclose the use of a non-electric injector discharging into the center of the "floating" Venturi structure. Such a single-point, continuous injection system confers the benefits of one large injector into the low pressure, high velocity air stream with time to disperse, evaporate and homogenize the air-fuel mixture before entry into combustion regions.
All fuel injection systems include air-flow metering devices to control the quantity of fuel discharged through throttle flow devices called "injectors." These devices transform liquid fuel into a spray of finely divided particles or droplets which commingle with combustion air in an engine to form an ignitable mixture. These are referred to as injection nozzles and electric-injectors. Electric-injectors consist of a solenoid-operated valve which quantifies flow into a nozzle from a constant pressure source by the amount of time it is opened.
The nozzle function of discharging and breaking up fuel into engine combustion air is based upon the pressure energy dissipated therein and fuel quantity discharged is proportional to the pressure drop from inflow pressure to the prevailing air pressure at point of discharge. Therefore, injection nozzles of any design, sized for a specific application will discharge fuel quantity in proportion to pressure drop and considering that air pressures at discharge are very small relative to fuel pressures, fuel quantity discharged will vary proportionately to the fuel pressure applied.
Presently, all systems of fuel injection that effect "closed-loop" control of air-fuel ratio do so by means of an exhaust gas electronic sensor, the Lambda Oxygen Exhaust Sensor. This "Lambda" method measures the exhaust gas content after combustion then feeds back deviation from the norm to control of fuel quantity injected before combustion. This system can only average optimum ratio with wide variations resulting from the time span from exhaust sampling to fuel quantity injected.
The advantages of closed-loop control of air-fuel mixture ratio can only be obtained by the air-flow controlling fuel flow in real time before combustion to a predetermined ratio norm and the capability of adjusting the ratio norm to operating conditions. This method of control implies some form of "servo" system by which is inferred "feed-forward" of air-flow into the engine and "feed-back" of fuel flow or their equivalents in real time before combustion.